1. Field of the Invention
This invention relates to an image processing apparatus, an image acquisition method and a program which are desirably applied to the biometrics authentication, for example.
2. Description of the Related Art
As one of the biological characteristics, a blood vessel draws attention. An apparatus, which utilizes a blood vessel to generate data to be registered (referred to as registration data, hereinafter) or data to be collated (referred to as collation data, hereinafter) with respect to the registration data, generally picks up an image of a blood vessel at certain part of biological body, and, with respect to image data acquired as the image pickup result, performs various image processings to extract a formation pattern of blood vessel (referred to as blood vessel pattern, hereinafter) contained in the image data.
When picking up an image of a blood vessel, a near infrared ray is employed as light for the image pickup. Since a near infrared ray is specifically absorbed by deoxygenation hemoglobin (venous blood) or oxygenation hemoglobin (arterial blood) in a vessel tissue, a near infrared ray which is reflected and scattered at the inside of part of biological body and sent from the part of biological body comes to be a blood vessel projection light that projects a blood vessel. As shown in FIG. 10, a picked up image is substantially monotone, and the luminance value of pixels corresponding to a blood vessel pattern is acquired as a low value, while the luminance value of pixels corresponding to part of biological body other than the blood vessel pattern is acquired as a high value.
On the other hand, since blood vessels are in the inside of a biological body, the difference between the luminance value of pixels corresponding to a blood vessel pattern and the luminance value of pixels corresponding to part of biological body other than the blood vessel pattern is not obvious, that is, the blood vessel pattern becomes blurred.
In general, this blurred state is made clear by the contour extraction processing etc. On the other hand, due to the individual difference, since this processing is meaningless when a blood vessel pattern is not picked up, it is necessary to adjust the image pickup conditions such as the light amount of a near infrared ray and the exposure value of an image pickup means (camera) according to the individual difference.
Among these image pickup conditions, when paying attention to the exposure value of an image pickup means (camera), there has been suggested an exposure control method in Jpn. Pat. Appln. Laid-Open Publication No. 2005-260738 under which a photometry value of respective areas obtained by dividing a picked up image is weighted to calculate a temporary exposure value, and an exposure compensation amount is calculated based on the difference between a threshold value which is obtained by adding a predetermined value to the temporary exposure value and the maximum photometry value, and then the temporary exposure value is compensated using the exposure compensation amount.